Apart from those sort of nouns/substantives, I can't find an example of a present participle as a vocative.

What sort ?You mean such participles as appear as substantives in dictionaries, like oriens under orior ?Do you say a present participle like sumens, which has no appearance as a substantive under sumo in the dictionary, would not be used in vocative ?

Usually the present participle qualifies "tu" in the nominative where there is a vocative near.

Then, is it possible that a pres. part. in voc. is attached to a hiding noun in vocative ?

Anyway, this sumens illud ave Gabrielis ore and funda nos in pace are united in one sentence in a chronically strange manner, so awkwardly connect to each other.I found that even with sumens taken as vocative awkwardness doesn't disappear, as well as the bnominative sumens in adverbial (ablatival) meaning.Maybe I should grasp the sentence more loosely as a kind of poetical grammar ?